[gui] add explicit prune setter

This makes it possible to enable pruning after the OptionsModel has been initialized and reset.

src/qt/optionsmodel.h

@@ -77,6 +77,9 @@ public:
     bool getCoinControlFeatures() const { return fCoinControlFeatures; }
     const QString& getOverriddenByCommandLine() { return strOverriddenByCommandLine; }
 
+    /* Explicit setters */
+    void SetPrune(bool prune, bool force = false);
+
     /* Restart flag helper */
     void setRestartRequired(bool fRequired);
     bool isRestartRequired() const;